1. Field of the Invention
The present invention relates to a vibration damping device adapted for installation between components to be provided with vibration damped linkage, and particularly to a fluid filled type vibration damping device that utilizes vibration damping action based on the flow action of a sealed therein.
2. Description of the Related Art
In the field of vibration damping devices such as vibration damping linkages or vibration damping supports designed for installation between components that make up a vibration transmission system, one type of known device is a fluid filled type vibration damping device that utilize vibration damping action based on the flow action of a fluid sealed inside. Such a fluid filled type vibration damping device has a first mounting member and a second mounting member linked by a rubber elastic body. To the inner peripheral side of the second mounting member, there is defined a pressure receiving chamber whose wall is partially constituted by the main rubber elastic body, and an equilibrium chamber whose wall is constituted by a flexible film. The chambers are filled with a non-compressible fluid, and the pressure receiving chamber and the equilibrium chamber communicate with each other through an orifice passage.
Based on the flow action of a fluid induced to flow between the pressure receiving chamber and the equilibrium chamber through the orifice passage, excellent vibration damping action can be produced against vibration in a specific targeted frequency range. The application of such fluid filled type vibration damping devices in automotive engine mounts, body mounts, and suspension mounts for example, is a topic of ongoing research.
In the fluid filled type vibration damping device of this kind, while excellent vibration damping effect may be attained at times of input of vibration in frequency range to which the orifice passage has been pre-tuned, it may suffer from the problem effective vibration damping action will not be attained upon input of in other frequency ranges. Since a fluid filled type vibration damping device will in some instances experience simultaneous input of vibration in multiple frequency ranges, and particularly given the current increasingly higher requirements for vibration damping capabilities, there accordingly exists a need for a fluid filled type vibration damping device that is capable of exhibiting effective vibration damping effect against vibration over a wider frequency range.
Moreover, fluid filled type vibration damping devices also have the problem of noise and vibration being produced at times of sudden input of large impact load. Specifically, where a fluid filled type vibration damping device is employed as an automotive engine mount for example, if the car happens to drive over a grooved roadway having grooves and ridges, in some instances the noise and vibration will be sufficient to be noticeable to passengers.
A phenomenon known as cavitation, produced by sharp pressure fluctuations within the pressure receiving chamber, may be cited as a cause of such noise and vibration. Specifically, with sudden input of large impact load, the main rubber elastic body that defines a wall of the pressure receiving chamber will experience appreciable elastic deformation and produce a marked drop in liquid pressure within the pressure receiving chamber, whereupon air bubbles known as cavitation will be formed. Water hammer pressure created as the bubbles burst will be transmitted to the vehicle via the vibration damping device, producing noise and vibration of an extent posing problems within the cabin.
In order to prevent the occurrence of such noise and vibration caused by cavitation, there has been proposed, for example, in U.S. Pat. No. 4,781,362, a fluid filled type vibration damping device having a structure with a slit formed in a rubber film that has been disposed so as to partition the pressure receiving chamber and the equilibrium chamber which are filled with non-compressible fluid. Specifically, in the fluid filled type vibration damping device disclosed in U.S. Pat. No. 4,781,362, when excessive negative pressure arises in the pressure receiving chamber, the rubber film will be suctioned towards the pressure receiving chamber side and undergo elastic deformation, causing the slit that was formed in the rubber film to open up, so that the pressure receiving chamber and the equilibrium chamber communicate with each other through the slit. The negative pressure in the pressure receiving chamber is quickly dispelled thereby, preventing noise and vibration caused by cavitation.
However, the fluid filled type vibration damping device disclosed in U.S. Pat. No. 4,781,362 still leaves a number of problems unsolved. First, if a slit is made in the rubber film separating the pressure receiving chamber and the equilibrium chamber as taught in U.S. Pat. No. 4,781,362, the slit will open up not only at times when negative pressure acts on the pressure receiving chamber, but also when positive negative pressure acts on the pressure receiving chamber, thus creating a risk of short circuiting of the pressure receiving chamber and the equilibrium chamber through the slit. Consequently, even at times of input of ordinary load of the kind that has been targeted for vibration damping, escape of pressure from the pressure receiving chamber to the equilibrium chamber may reduce the amount of fluid induced to flow through the orifice passage, with the risk that the intended vibration damping effect may not be produced on the basis of the flow action of the fluid.
One conceivable means for solving such problems would be to increase the rigidity of the rubber film, e.g. by making it thicker, so as to limit elastic deformation of the film and make it harder for the slit to open up. However, if the rigidity of the rubber film is increased to a sufficient extent, while the desired vibration damping effect will be attained, there is also a risk that the slit will fail to open by a sufficient amount at times when excessive negative pressure necessitating opening of the slit arises, thus diminishing the effect of reducing noise and vibration as discussed above.
Another conceivable approach would be to make the slit smaller in size to make it harder for the slit to open up, so as to attain the required vibration damping capability at times of input of ordinary load. However, making the slit smaller poses the risk of inability to effectively obtain the negative pressure dispelling effect that will be needed when excessive negative pressure arises in the pressure receiving chamber. That is, in instances where the input load is abnormally large and a very high level of negative pressure has arisen in the pressure receiving chamber, the negative pressure within the pressure receiving chamber will not be dispelled sufficiently, due to insufficient opening area of the slit, thus posing the risk of occurrence of noise and vibration caused by cavitation.
That is, according to the fluid filled type vibration damping device disclosed in U.S. Pat. No. 4,781,362, it was difficult to effectively achieve both the aim of reducing or eliminating noise and vibration caused by cavitation, and the inherent vibration damping capabilities required of the vibration damping device.
Depending on a structure for a fixation of the rubber film, etc., the rubber film may be exposed to excess pressure. As a result, the fixation position of the rubber film may be dislocated, or the rubber film may fail to restore its initial shape, making it impossible for exhibiting desired vibration damping effect and for reducing noises, with stability.